1. Field
This invention relates to plasma ion implant, such as used for fabrication of semiconductor devices, particularly solar cells.
2. Related Arts
Diffused junction solar cells are known in the art. Such cells are fabricated by doping a silicon substrate with dopants such as boron and phosphorous. Metallic contacts are then fabricated to collect current generated by the cell. Also known are cells referred to front contact cells where selective n++ emitter can be formed, wherein high doping concentration is provided at the area of the emitter's surface where the metallic contacts are deposited. Generally, blanket doping is done using POCl3 diffusion. Another known solar cell, in back contact cell that is generally referred to as point contact or interdigitated back-contact, or IBC, cell, wherein all of the electrical contacts are provided only on the backside of the solar cell. The selective emitter and IBC cells requires both a blanket doping of the entire sun-facing area of the wafer and selective doping only on selected areas of the backside of cell. For example, for selective emitter the areas that form the contact to the metal lines on the sun-facing surface of the solar cell are selectively doped at a higher dose than the background blanket doping. On the other hand, IBC cells require alternating areas of p-type and n-type doped areas, that are generally formed in separate steps using, e.g., masks.
The requirement for IBC masks to be able to provided varying doped patterned implant region is very rigorous. The opening in such mask can be a range such 200 to 1000 micron, and traverse the whole length of the substrate. The spacing or pitch of these line, for each dopant, can a range less than 1000 micron, where the lines for the opposing doped line can land in between and positioned to high precision. Fabrication of such a mask, may render it non-rigid and difficult to make, handle and use in an ion implant system. Any means of cross stitching the openings will lead to shadowing of the doped lines and hence render the doped line with poor or intermittent conductivity.
In addition there is a requirement for the selective implantation to be in two dimensional, where there are patterns that are in both x and y direction across the surface of the substrate. These could include lines with corners, deviation, shaped to accommodate the current flow, circles, halo, dots and other 2D shapes.
While it has been proposed that ion implantation can result in better doping profiles and, therefore, better cell efficiency, ion implantation has not been used due to the high cost and slow throughput of standard ion implantation equipment. However, with the increased demand for improved cell efficiency and for formation of selective emitter or IBC cells, market demand developed for ion implant technologies that can deliver the required low cost and high throughput. Consequently, certain solutions are emerging, which are plasma-based ion implant. One solution is generally referred to as plasma immersion ion implantation, PIII, in which plasma engulfs that substrate to be implanted. A related solution is to insert a grid assembly between the plasma and the substrate, such that ion are extracted from the plasma and are implanted onto the substrate, without the plasma contacting the substrate. While this solution seems promising, it requires modifications and improvement in order to enable selecting implantation.